Among currently-employed processes for synthesizing acetic acid one of the most useful commercially is the carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al. on Oct. 30, 1973. The catalyst comprises rhodium, either dissolved or otherwise dispersed in the liquid reaction medium or else supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by, for example, methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and it is not relevant, if indeed it is possible, to identify the exact nature of the rhodium moiety within the active catalyst complex. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. These compounds are employed as promoters, not stabilizers. Most typically and usefully, the reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled.
Paulik et al. teach that the liquid reaction medium can be any solvent compatible with the catalyst system and that it may comprise, for example, the pure alcohol which is being reacted, or mixtures thereof with the desired carboxylic acid end product and/or esters of these two compounds. However, the patentees teach further that the preferred solvent and liquid reaction medium for the process is the desired carboxylic acid itself, i.e., acetic acid when methanol is being carbonylated to produce acetic acid.
An important aspect of the teachings of Paulik et al. is that water should also be present in the reaction mixture in order to attain a satisfactorily high reaction rate. The patentees exemplify a large number of reaction systems including a large number of applicable liquid reaction media. The general thrust of their teachings is, however, that a substantial quantity of water helps in attaining an adequately high reaction rate. The patentees teach furthermore that reducing the water content leads to the production of ester product as opposed to carboxylic acid. Considering specifically the carbonylation of methanol to acetic acid in a solvent comprising predominantly acetic acid and using the promoted catalyst taught by Paulik et al., it is taught in European Patent Application No. 0055 618 that typically about 14-15 wt % water is present in the reaction medium of a typical acetic acid plant using this technology. It will be seen that in recovering acetic acid in anhydrous or nearly anhydrous form from such a reaction solvent, separating the acetic acid from this appreciable quantity of water, involves substantial expenditure of energy in distillation and/or additional processing steps such as solvent extraction, as well as enlarging some of the process equipment as compared with that used in handling drier materials. Also Hjortkjaer and Jensen [Ind. Eng. Chem., Prod. Res. Dev. 16, 281-285 (1977)] have shown that increasing the water from 0 to 14 wt % water increases the reaction rate of methanol carbonylation. Above 14 wt % water the reaction rate is unchanged.
In addition, as will be further explained hereinbelow, the catalyst tends to precipitate out of the reaction medium as employed in the process of Paulik et al., especially during the course of distillation operations to separate the product from the catalyst solution when the carbon monoxide content of the catalyst system is reduced (EP0055618). It has now been found that this tendency increases as the water content of the reaction medium is decreased. Thus, although it might appear obvious to try to operate the process of Paulik et al. at minimal water concentration in order to reduce the cost of handling reaction product containing a substantial amount of water while still retaining enough water for adequate reaction rate, the requirement for appreciable water in order to maintain catalyst activity and stability works against this end.
Other reaction systems are known in the art in which an alcohol such as methanol or an ether such as dimethyl ether can be carbonylated to an acid or ester derivative using special solvents such as aryl esters of the acid under substantially anhydrous reaction conditions. The product acid itself can be a component of the solvent system. Such a process is disclosed in U.S. Pat. No. 4,212,989 issued Jul. 15, 1980 to Isshiki et al., with the catalytic metal being a member of the group consisting of rhodium, palladium, iridium, platinum, ruthenium, osmium, cobalt, iron, and nickel. A somewhat related patent is U.S. Pat. No. 4,336,399 to the same patentees, wherein a nickel-based catalyst system is employed. Considering U.S. Pat. No. 4,212,989 in particular, the relevance to the present invention is that the catalyst comprises both the catalytic metal, as exemplified by rhodium, along with what the patentees characterize as a promoter, such as the organic iodides employed by Paulik et al. as well as what the patentees characterize as an organic accelerating agent. The accelerating agents include a wide range of organic compounds of trivalent nitrogen, phosphorus, arsenic, and antimony. Sufficient accelerator is used to form a stoichiometric coordination compound with the catalytic metal. Where the solvent consists solely of acetic acid, or acetic acid mixed with the feedstock methanol, only the catalyst promoter is employed (without the accelerating agent), and complete yield data are not set forth. It is stated, however, that in this instance "large quantities" of water and hydrogen iodide were found in the product, which was contrary to the intent of the patentees.
European Published Patent Application No. 0 055 618 to Monsanto Company discloses carbonylation of an alcohol using a catalyst comprising rhodium and an iodine or bromine component wherein precipitation of the catalyst during carbon monoxide-deficient conditions is alleviated by adding any of several named stabilizers. A substantial quantity of water, of the order of 14-15 wt %, was employed in the reaction medium. The stabilizers tested included simple iodide salts, but the more effective stabilizers appeared to be any of several types of specially-selected organic compounds. There is no teaching that the concentrations of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid especially at low water concentrations. When an iodide salt is used as the stabilizer, the amount used is relatively small and the indication is that the primary criterion in selecting the concentration of iodide salt to be employed is the ratio of iodide to rhodium. That is, the patentees teach that it is generally preferred to have an excess of iodine over the amount of iodine which is present as a ligand with the rhodium component of the catalyst. Generally speaking the teaching of the patentees appears to be that iodide which is added as, for example, an iodide salt functions simply as a precursor component of the catalyst system. Where the patentees add hydrogen iodide, they regard it as a precursor of the promoter methyl iodide. There is no clear teaching that simple iodide ions as such are of any significance nor that it is desirable to have them present in substantial excess to increase the rate of the reaction. As a matter of fact Eby and Singleton [Applied Industrial Catalysis, Vol. 1, 275-296(1983)] from Monsanto state that iodide salts of alkali metals are inactive as cocatalyst in the rhodium-catalyzed carbonylation of methanol.
Carbonylation of esters, such as methyl acetate, or ethers, such as dimethyl ether, to form a carboxylic acid anhydride such as acetic anhydride is disclosed in U.S. Pat. No. 4,115,444 to Rizkalla and in European Patent Application No. 0,008,396 by Erpenbach et. al. and assigned to Hoechst. In both cases the catalyst system comprises rhodium, an iodide, and a trivalent nitrogen or phosphorus compound. Acetic acid can be a component of the reaction solvent system, but it is not the reaction product. Minor amounts of water are indicated to be acceptable to the extent that water is found in the commercially-available forms of the reactants. However, essentially dry conditions are to be maintained in these reaction systems.
U.S. Pat. No. 4,374,070 to Larkins et al. teaches the preparation of acetic anhydride in a reaction medium which is, of course, anhydrous by carbonylating methyl acetate in the presence of rhodium, lithium, and an iodide compound. The lithium can be added as lithium iodide. Aside from the fact that the reaction is a different one from that with which the present invention is concerned, there is no teaching that it is important per se that the lithium be present in any particular form such as the iodide. There is no teaching that iodide ions as such are significant.
In summary, current technology in the field of carbonylating an alcohol such as methanol to form a carboxylic acid such as acetic acid still lacks a simple method for maintaining a highly stable catalyst system and for attaining industrially attractive conversion rates under conditions of low water content in the liquid reaction medium whereby the expense and capital investment costs of recovering the acid product with a very low water content can be minimized.
It is, accordingly, an object of the present invention to provide a reaction system with which an alcohol, as exemplified by methanol, can be carbonylated to a carboxylic acid derivative such as acetic acid while using a liquid reaction medium having a lower water content than heretofore considered feasible. It is another object to provide a catalyst system which, regardless of the water content of the reaction medium, will be of improved stability--i.e., more resistant to precipitation of solid catalyst therefrom. It is also a related object to provide a catalyst system characterized by a substantial reduction in the undesired formation of by-product propionic acid, carbon dioxide, and hydrogen as compared with high water systems used in the prior art. Other objects will be apparent from the following detailed description.